1. Field of the Invention
The present invention relates to a nitride semiconductor device, and more particularly to a nitride semiconductor device having improved current diffusion and increased surface roughness, whereby electric and optical characteristics are improved. Also, the present invention relates to a method of manufacturing the same.
2. Description of the Related Art
Generally, a nitride semiconductor is widely used as a material for manufacturing visible-light, ultraviolet-light, and cyan optical devices, such as a light emitting diode (LED) or a laser diode (LD), since the nitride semiconductor has the characteristics of emitting light having a wide range from visible light to ultraviolet light. The nitride semiconductor is a semiconductor single crystal satisfying the following formula: AlxInyGa(1-x-y)N (where, 0≦x≦1, 0≦y≦1, and 0≦x+y≦1). The nitride semiconductor is grown on a substrate, such as sapphire or SiC, by means of a crystal growth process, such as Metal Organic Chemical Vapor Deposition (MOCVD).
Principally, a nitride-based semiconductor device comprises an n-type nitride semiconductor layer, an undoped active layer, and a p-type nitride semiconductor layer. A conventional nitride semiconductor device 10 is shown in FIG. 1.
Referring to FIG. 1, the conventional nitride semiconductor device 10 comprises a sapphire substrate 11 having a buffer layer 12, such as a GaN or AlN low-temperature core growth layer, formed thereon. On the buffer layer is formed an n-type nitride semiconductor layer 13. On the n-type nitride semiconductor layer 13 is formed an undoped active layer 14. On the undoped active layer 14 is formed a p-type nitride semiconductor layer 15. To the n-type nitride semiconductor layer 13 and the p-type nitride semiconductor layer 15 are connected n-side and p-side electrodes 18 and 19, respectively. The active layer 14 has a multi-quantum well structure (MQW) in which quantum barrier layers made of GaN and quantum well layers made of InGaN are alternately stacked several times.
When predetermined current is applied between the two electrodes 18 and 19, electrons provided from the n-type nitride semiconductor layer 13 are recombined with holes provided from the p-type nitride semiconductor layer 15 in the active layer 14 having the multi-quantum well structure. As a result, light having a desired wavelength, such as a green wavelength or a blue wavelength, is emitted.
In order to improve light efficiency of the nitride semiconductor device, brisk studies into improving inner quantum efficiency and outer quantum efficiency (i.e., light extraction efficiency) have been made. To improve the inner quantum efficiency, it is necessary to increase light efficiency generated from the active layer. In this case, the structure of the active layer 14 and the crystal qualities of the epitaxial layers 13, 14, and 15 are focused.
The inner quantum efficiency is seriously limited due to ununiform current diffusion. As shown in FIG. 1, electric current is concentrated to a part A of the active layer 14 with the result that the other parts of the active layer 14 have a relatively low current density. Consequently, the entirety of the active layer does not serve as a light-emitting region, which decreases the inner quantum efficiency. Up to now, improvement of electrode arrangement and a p-side electrode structure has focused on ensuring uniform current diffusion.
Also, a refractive index and surface flatness of a semiconductor material may be controlled to improve the outer quantum efficiency (i.e., light extraction efficiency). However, the refractive index of the nitride semiconductor has a limited changeable range. As a result, the outer quantum efficiency is improved just a little. To control the surface flatness, it is necessary to increase surface roughness such that a total reflection angle is decreased in the device, and thus the amount of light lost in the device is decreased. However, it is necessary to perform pattern formation through the use of Metal Organic Chemical Vapor Deposition (MOCVD) or other CVD processes in order to increase the surface roughness, which is very troublesome.
As described above, various efforts to improve the light efficiency of the nitride semiconductor device have been made, and it is still necessary to improve the electric and optical characteristics, and thus the light efficiency, by means of more effective methods.